The invention is in the field of inverter circuits employing transistors as switches for producing a-c output energy from d-c input energy.
A widely used type of inverter circuit, known as a half-bridge inverter, has a pair of transistors, the emitter of one of the transistors and the collector of the other transistor being connected together to form a junction point, the remaining emitter and collector being respectively connected to terminals of a d-c electrical power source. A load (such as a lamp, motor, induction heater or other suitable device) is connected between the aforesaid junction point and the junction point of two capacitors connected in series across the power source. The transistors are alternately driven to full conduction at a repetitive frequency rate of about 1 kHz or higher, by means of drive currents alternately applied to their base electrodes, thus passing an alternating current through the load from the power source. The aforesaid drive currents may be square waves or sine waves, of sufficient amplitude to drive the transistors quickly to full conduction whereby they add substantially no resistance in the power current path and thus do not consume and dissipate any appreciable electrical power.
U.S. Pat. No. 4,051,426 to Wood discloses an inverter circuit of the type just described, and also discloses an undesirable "stored charge" characteristic of transistors which can cause both of the inverter transistors to be conductive at the same time and produce an undesirable short circuit across the power source, known as "shoot-through". The patent also teaches a way of connecting diodes in the circuit to alleviate the problem.
The first above-referenced Anderson et al patent application, which is incorporated herein by reference thereto, is directed to a circuit which makes advantageous use of stored charges in inverter transistors. The stored charges are produced from inductively induced flyback current in a load which is inductive or partly inductive. Each time one of the transistors is turned off, the flyback current, which is induced by declining current in the load, is applied to the base electrode of the other transistor to create a stored charge for turning it on and keeping it on during a half-cycle of operation. Thus the transistors are alternately conductive and apply alternating current through the load.